Many medical devices adapted for implantation also have high power requirements and must be frequently connected to external power sources. Inductively coupled transcutaneous energy transfer (TET) systems are increasingly popular for use in connection with these high-power implantable devices. A TET system may be employed to supplement, replace, or charge an implanted power source, such as a rechargeable battery. Unlike other types of power transfer systems, TET systems have an advantage of being able to provide power to the implanted electrical and/or mechanical device, or recharge the internal power source, without puncturing the skin. Thus, possibilities of infection are reduced and comfort and convenience are increased.
TET devices include an external primary coil and an implanted secondary coil, separated by intervening layers of tissue. The primary coil is designed to induce alternating current in the subcutaneous secondary coil, typically for transformation to direct current to power an implanted device. TET devices therefore also typically include an oscillator and other electrical circuits for providing appropriate alternating current to the primary coil. These circuits typically receive their power from an external power source.
TET systems commonly include an implanted rechargeable battery pack that can be used to power any implanted devices when the external power source is not available. However, when disconnected from the external power source patients are often unsure of how long the internal battery pack will last before requiring a charge cycle. Prior art methods of calculating remaining charge are based on the battery voltage, not the actual remaining charge. Because the battery voltage is not linear with respect to charge, these methods can report a near total charge for a long period of time and then quickly approach total exhaustion rather than report a linear decrease over time. This misleading indication of battery charge can be extremely risky for patients that depend on their implanted TET system for survival and who may not be able to immediately reconnect to an external power or charge source.